1. Field of the Invention
This invention relates to a fine powder of a marking agent and to a marking method using the marking agent and, more particularly, to an improved marking agent and method wherein clear durable marking patterns can be formed even on slightly heated or highly heated members as well as on unheated members.
2. Description of the Prior Art
As is well known in the art, steel members which are continuously produced in steel-making plants must be marked with necessary information to control the production process, to prevent different steel members from being mixed together during transportation, and to indicate the destinations of the steel members. Since steel members, particularly in continuous hot rolling mills are fed from the furnace while red hot or at a similar high temperature, adhesion of labels or the like to such members is almost impossible and thus a marking method using heat-resistant marking agents is required.
One of the most widely accepted marking methods is that which comprises holding a tin stencil, which has been previously made, on a steel member to be marked and spraying a heat-resistant paint through the stencil thereby marking the desired information on the steel member. However, this method must be done by hand work and it is thus undesirable from a standpoint of efficiency. In addition, a large number of stencils, which require a lot of time to make, must be available and the stencils have a relatively poor durability. As a result, great difficulty is encountered in having the required number of stencils available at all times.
Recently, in order to prevent working difficulties and facilitate a saving of labor, a number of marking methods have been proposed which comprise making a single-use stencil from paper using instructions from a computer, and controlling an automatic powder coating operation through the stencil and opening or closing a number of nozzles by means of the computer, thereby spraying the powder coating onto a member from a spray nozzle. Some of these marking methods are now employed into practice.
Marking methods based on electrostatic recording techniques using powdery marking agents are disclosed in our Japanese Patent Application Nos. 149244/1975 and 42090/1976.
In general, a hot rolled member to be marked is fed to the marking station at temperatures of from room temperature (about 10.degree. C. to 20.degree. C.) to about 400.degree. C. Conventionally employed marking agents as described in Keisoku Gijutsu (metering Techniques) August pp 62-70 (1974), Japanese Patent Publication No. 17877/1972, etc. are those which are commercially available, or are similar in composition to commercially available powder coatings, and have the following drawbacks in common.
The marking agents are thermally decomposed to some degree before a hot member at above about 350.degree. C., especially a thick member, cools to normal temperature. As a consequence, the final marking pattern has a low optical contrast and is so mechanically fragile that the pattern tends to be broken down by forces received during working operations such as transfer.
On the other hand, marking agents with very high heat-resistance as described in Japanese Patent Application (OPI) Nos. 104046/1975 and 105724/1975, Japanese Patent Publication Nos. 24406/1972 and 17496/1974, etc. have excessively high melting points, so that it is very difficult to adhere the agents on a member even though heated, which is at a temperature below the melting points thereof.